Field
This invention relates generally to a digital-to-analog converter (DAC) that includes a clock tree distribution network and, more particularly, to a DAC including a DAC core circuit and a clock tree distribution network, where the clock tree distribution network is fabricated in SiGe and the DAC core circuit is fabricated in InP.
Discussion
There are many applications that require a digital-to-analog converter (DAC), such as converting digitally stored audio signals to a representative analog signal that is suitable to drive an audio amplifier and a speaker. A typical DAC will include an array of converter cells that each receive a digital data signal and convert the signal to a representative voltage and/or frequency that is combined with the voltage and/or frequency from the other cells through a current switching network. It is necessary to provide accurate clock signal timing between the current switching networks in the various cells to achieve a high dynamic range. Particularly, clock misalignment or skew across the DAC cell array typical translates into unwanted spurs in the DAC analog output signal. However, maintaining signal integrity throughout the switching network becomes more difficult at higher frequencies. A digital clock tree distribution network is a known clock signal distribution circuit for DACs and is currently the most common solution for minimizing clock skew across a DAC cell array.
Modern DACs of the type discussed above are fabricated as integrated circuits using various semiconductor materials. As is well understood by those skilled in the art, different semiconductor materials offer different properties and advantages for a DAC. Semiconductor material trade-offs are typically made between maximum clock frequency, clock skew, signal integrity and power dissipation. For example, it is known in the art to fabricate DAC core and clock tree distribution circuits from either indium phosphide (InP) or silicon germanium (SiGe). InP provides a higher voltage breakdown, which provides higher output voltages that may be beneficial for defining over noise. However, InP does not allow devices in the circuit to be closely spaced together, which increases clock skew and signal mismatch. SiGe does not provide a high voltage breakdown and voltage output, but does allow for a reduction in clock skew as a result of the devices being more closely space together.